CIRCULATION OF THE BLOOD. 557 



is midway between extreme constriction and dilation. The flow through 

 A is determined by the resistance in A and in the vascular tract which it 

 supplies, in relation to the mean arterial pressure, which again is depend- 

 ent on the way in which the heart is beating and on the peripheral re- 

 sistance of all the small arteries and capillaries, A included. If, while 

 the heart and the rest of the arteries remain unchanged, A be constricted, 

 the peripheral resistance in A will increase, and this increase of resist- 

 ance will lead to an increase of the general arterial pressure. This in- 

 crease of pressure will tend to cause the blood in the body at large to 

 flow more rapidly from the arteries into the veins. The constriction of 

 A, however, will prevent any increase of the flow through it in fact, will 

 make the flow through it less than before. Hence, the whole increase of 

 the discharge from the arterial into the venous system must take place 

 through channels other than A. Thus, as the result of the constriction 

 of any artery there occur (1) diminished flow through the artery itself, 

 (2) increased general arterial pressure, leading to (3) increased flow 

 through the other arteries. If, on the other hand, A be dilated, while 

 the heart and other arteries remain unchanged, the peripheral resistance 

 in A is diminished. This leads to a lowering of the general arterial pres- 

 sure, which, in turn, causes the blood to flow less rapidly from the arteries 

 into the veins. The dilation of A, however, permits, even with the low- 

 ered pressure, more blood to pass through it than before. Hence, the 

 diminished flow tells all the more on the rest of the arteries. Thus, as 

 the result of the dilation of any artery, there occur (1) increased flow of 

 blood through the artery itself, (2) diminished general pressure, and (3) 

 diminished flow through the other arteries. Where the artery thus con- 

 stricted or dilated is small, the local effect, the diminution or increase of 

 flow through itself, is much more marked than the general effects, the 

 change in blood pressure and the flow through other arteries. When, 

 however, the area the arteries of which are affected is large, the general 

 effects are very striking. Thus, if while a tracing of the blood pressure 

 is being taken by means of a manometer connected with the carotid 

 artery the splanchnic nerve be divided, a conspicuous but steady fall of 

 pressure is observed very similar to that which is seen in Fig. 235. The 

 section of the splanchnic nerves causes the mesenteric and other abdomi- 

 nal arteries to dilate, and these being very numerous a large amount of 

 peripheral resistance is taken away, and the blood pressure falls accord- 

 ingly ; a large increase of flow into the portal veins takes place, and the 

 supply of blood to the face, arms, and legs is proportionally diminished. 

 It will be observed that the dilation of the arteries is not instantaneous, 

 but somewhat gradual, the pressure sinking, not abruptly, but with a 

 gentle curve. 



Arterial tone, then, both general and local, is a powerful instrument 



